The present invention is generally directed to the construction of liquid crystal display devices. More particularly, the present invention is directed to a liquid crystal display structure which incorporates spacer material which also performs a light blocking function.
A liquid crystal display device typically comprises a pair of flat panels sealably containing a quantity of liquid crystal material. These liquid crystal materials typically fall into two categories: dichroic dyes in a guest/host system or twisted nematic materials. The flat panels generally possess transparent electrode material disposed on their inner surfaces in predetermined patterns. One panel is often covered completely by a single transparent "ground plane" electrode. The opposite panel is configured with an array of transparent electrodes, referred to herein as "pixel" (picture element) electrodes. Thus, the typical cell in a liquid crystal display includes liquid crystal material disposed between a pixel electrode and a ground electrode forming, in effect, a capacitor-like structure disposed between transparent front and back panels. In general, however, transparency is only required for one of the two panels and the electrodes disposed thereon.
In operation, the orientation of liquid crystal material is affected by voltages applied across the electrodes on either side of the liquid crystal material. Typically, a voltage applied to the pixel electrode effects a change in the optical properties of the liquid crystal material. This optical change causes the display of information on the liquid crystal display (LCD) screen. In conventional digital watch displays and in newer LCD display screens used in miniature television receivers, the visual effect is typically produced by variations in reflected light. However, the utilization of transparent front and back panels and transparent electrodes also permit the visual effects to be produced by transmissive effects. These transmissive effects may be facilitated by separately powered light sources for the display, including fluorescent light type devices. LCD display screens may also be employed to produce color images through the incorporation of color filter mosaics in registration with the pixel electrode array. Some of these structures may employ polarizing filters to either enhance or provide the desired visual effect.
Various electrical mechanisms are employed to sequentially turn on and off individual pixel elements in an LCD display. For example, metal oxide varistor devices have been employed for this purpose. However, the utilization of thin film semiconductor switch elements is most relevant herein. In particular, a preferable switch element comprises a thin film field effect transistor (FET). These devices are preferred in LCD displays because of their potentially small size, low power consumption, switching speeds, ease of fabrication, and compatibility with conventional LCD structures. However, some semiconductor switch devices, notably thin film FETs, exhibit an undesirable degree of light sensitivity. This is undesirable because the nature of the device typically requires either ambient light or a built in light source. These light sources can act to cause charge leakage between the ground plane and the pixel electrodes. This produces undesirable visual effects on the display screen.
More particularly, amorphous silicon FET addressed liquid crystal matrix displays provide an attractive approach to high contrast, flat panel television type displays. Ideally, in an FET addressed LCD device, when the FET is turned on, the "liquid crystal capacitor" charges to the data or source line voltage. When the FET is turned off, the data voltage is stored on the liquid crystal capacitor. Amorphous silicon FETs however, are very photosensitive. Light absorbed in the channel region of the FET causes a leakage current to flow between the source and the drain. This leakage current causes the voltage stored on the liquid crystal capacitor to change, thereby degrading display performance. Since displays are often required to operate in high ambient light conditions, a means of keeping the light from affecting the FET is required. In conventional FET structures, a metallic gate electrode is formed on a transparent substrate. This gate electrode prevents light from reaching the FET channel region from the substrate side. In order to block light from the opposite (top side) an additional structure is required. Previously employed mechanisms to accomplish this have included the formation of a second gate electrode. However, this solution adds undesirable complexity to the structure. Additionally, other solutions employed have included the use of an electrically floating metal layer spaced above the FET channel and separated by a thick insulation layer. However, this adds unwanted capacitance between the source and drain. Even further attempts at solving light blocking problems have included the use of polymer material, but such material has not been employed anywhere except in the region of the FET device and certainly has not been employed to function as a spacer. Accordingly, an insulating, light blocking layer that does not add process complexity is desired.
In order to achieve uniform optical appearance, the thickness of an LCD cell, which is typically 3 to 15 microns, must be uniform over the cell to within .+-.O.2 microns. Since low cost glass materials desired for use in such displays may deviate from flatness by tens of microns over the dimensions of a display, spacers must be distributed throughout the cell. The two walls of the cell are then forced into contact with the spacers by filling the cell with the volume of liquid crystal material that just fills the cavity when the walls are in such contact. Spacers typically used are short lengths of glass fibers distributed randomly over the cell. With an FET driven LCD, the thickness of the FET structure may be about 1 micron. If a fiber spacer were to land on the FET, the cell thickness would locally be greater than if the fiber landed elsewhere. Therefore, a structure is required with the cell spacers located only at predetermined locations.
The pixel elements in an LCD are typically arranged in a rectangular array of rows and columns. Each pixel electrode is associated with its own FET switch device. Each switch device is connected to a data line and a gate line. Electrical signals applied simultaneously to each of these lines permit each pixel to be addressed independently. Accordingly, the LCD is typically provided with a set of parallel data lines which can be made to address cells in a horizontal direction. Likewise, gate lines are provided for accessing cells in a vertical direction. In operation, the image on the LCD device may be refreshed at a rate which is typically approximately 60 Hz.
However, a third problem arises with respect to areas of the LCD device near the data lines of the matrix. If most of the display elements along data lines are on, then the rms voltage appearing on the data line is approximately the same as the supply voltage, V.sub.0. This voltage on the data line tends to turn on the liquid crystal material near the data line. Accordingly, the present invention is directed to a structure for blocking this unwanted light transmission by covering the region of the cell adjacent to the data line with a light blocking layer which also provides a spacing function and simultaneously provides a process structure which provides light blocking for the semiconductor switch devices.